The present disclosure relates to processes for cleaning mixing devices that are used to make polycarbonates and polycarbonate blends. The processes aim to increase the speed of the cleaning process, provide lower resin degradation, and allow for a higher overall extrusion rate. The disclosure also relates to purge compositions useful in such processes.
Extruders using a melt conveying screw accomplish a large portion of the melt processing of thermoplastic resins. The extruder can be used for compounding, molding, pelletization or forming films, sheets or profiles. Such extruders typically have a heated extrusion barrel and one or two screws revolving within the barrel to compress, melt, and extrude the resin through an orifice in an extrusion nozzle. The barrel is divided into several different zones, such as feed, transition, mixing, dispersion, and metering zones. When such machines are dedicated to making a single material in a single color, they can be very efficient. However, when the same machine is used to make a variety of materials and/or colors, there is a need to changeover or switch between materials (i.e. changeover process). Otherwise, impurities or residues created during the extrusion of the first, preceding resin can contaminate the second, succeeding resin. During this changeover process, the extruder produces a combination of the preceding material/color and the succeeding material/color. As a result, the combination may not meet desired specifications, e.g. compositional differences, a loss of clarity, a change in color or viscosity, or some other defect. In addition, the changeover process leads to a loss of material and a decrease in manufacturing efficiency.
These deficiencies have been addressed to some extent in several ways. For instance, a high viscosity or sticky resin has been used to push out the old material. While this can be effective, the high viscosity resin itself must be pushed out of the machine, leading to inefficiency. In other instances, fiber glass reinforced resins are used as a purge compound to clean out the extruder. While effective in some systems, this process again leaves the problem of removing the fiber glass resin itself.
In other instances, carboxylic acid salts, fatty acid soaps, ammonium salts, amines, alcohols, alcohol sulfate salts and other chemical compounds or mixtures have been used to help clean out (purge) the extruder during changeover. For example, common laundry detergents like WISK® detergent and TIDE® detergent (which contain alcohols and carboxylic acid salts) are commonly used as purge compounds both in compounding and sheet extrusion. These detergents can also have a pH of 9 to 12.
However, such chemical compounds cannot be used effectively when the subsequent material is a polycarbonate resin or a blend containing polycarbonate resin. Polycarbonate is very susceptible to degradation, such as a change in color (i.e. color formation), loss of molecular weight, reduced chemical and mechanical properties, and/or surface appearance defects (e.g. splay in molded parts), due to the catalytic effects of acid or base. The use of carboxylic acid salts, fatty acid soaps, ammonium salts, and amines cause such degradation in the resulting polycarbonate material. If even a small amount of such an unsuitable material, such as a high (greater than 9.0) pH detergent, remains in the feeder or extruder, the polycarbonate (PC) resin used to clean the machine as well as the new polycarbonate will be degraded and the melt viscosity will drop. Foaming can also occur. Therefore in addition to loss of machine production time during a purge, there is degradation of the new polycarbonate material to be made, which reduces its utility. Put another way, detergents containing such materials themselves become a source of contamination of the subsequent polycarbonate material in addition to degrading the PC purge resin.
Thus, there exists a need for processes and compositions to quickly changeover a screw-based melt processing machine from one material to a polycarbonate-containing material in a short period of time, with no degradation of the polymer. Desirably, the process/composition should be applicable for cleaning a wide variety of contaminants, especially colorants such as carbon black, titanium dioxide, anthraquinone and phthalocyanine-based pigments and dyes.